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[ValueTracking] Recognize that and(x, add (x, -1)) clears the low bit
This is a cleaned up version of a patch by John Regehr with permission. Originally found via the souper tool. If we add an odd number to x, then bitwise-and the result with x, we know that the low bit of the result must be zero. Either it was zero in x originally, or the add cleared it in the temporary value. As a result, one of the two values anded together must have the bit cleared. Differential Revision: http://reviews.llvm.org/D14315 llvm-svn: 252629
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; RUN: opt -S -instsimplify < %s | FileCheck %s | ||
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define i1 @test(i32 %a) { | ||
; CHECK-LABEL: @test | ||
; CHECK: ret i1 false | ||
%rhs = add i32 %a, -1 | ||
%and = and i32 %a, %rhs | ||
%res = icmp eq i32 %and, 1 | ||
ret i1 %res | ||
} | ||
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define i1 @test2(i32 %a) { | ||
; CHECK-LABEL: @test2 | ||
; CHECK: ret i1 false | ||
%rhs = add i32 %a, 1 | ||
%and = and i32 %a, %rhs | ||
%res = icmp eq i32 %and, 1 | ||
ret i1 %res | ||
} | ||
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define i1 @test3(i32 %a) { | ||
; CHECK-LABEL: @test3 | ||
; CHECK: ret i1 false | ||
%rhs = add i32 %a, 7 | ||
%and = and i32 %a, %rhs | ||
%res = icmp eq i32 %and, 1 | ||
ret i1 %res | ||
} | ||
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@B = external global i32 | ||
declare void @llvm.assume(i1) | ||
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; Known bits without a constant | ||
define i1 @test4(i32 %a) { | ||
; CHECK-LABEL: @test4 | ||
; CHECK: ret i1 false | ||
%b = load i32, i32* @B | ||
%b.and = and i32 %b, 1 | ||
%b.cnd = icmp eq i32 %b.and, 1 | ||
call void @llvm.assume(i1 %b.cnd) | ||
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%rhs = add i32 %a, %b | ||
%and = and i32 %a, %rhs | ||
%res = icmp eq i32 %and, 1 | ||
ret i1 %res | ||
} | ||
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; Negative test - even number | ||
define i1 @test5(i32 %a) { | ||
; CHECK-LABEL: @test5 | ||
; CHECK: ret i1 %res | ||
%rhs = add i32 %a, 2 | ||
%and = and i32 %a, %rhs | ||
%res = icmp eq i32 %and, 1 | ||
ret i1 %res | ||
} | ||
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define i1 @test6(i32 %a) { | ||
; CHECK-LABEL: @test6 | ||
; CHECK: ret i1 false | ||
%lhs = add i32 %a, -1 | ||
%and = and i32 %lhs, %a | ||
%res = icmp eq i32 %and, 1 | ||
ret i1 %res | ||
} |